A variety of sensitometer devices are known in the prior art to expose samples of photographic papers and films in a very precise manner for subsequent densitometric analysis in which the density of the image produced by such exposure is measured. Such devices and the methods practiced thereby must be capable of exposing photosensitive samples with a high degree of precision, accuracy and repeatability.
In known sensitometers, the radiant energy source typically is located far enough away from the sample that the sample is essential uniformly illuminated and still receives enough radiant energy for proper exposure. Exposure times are created by a variety of shutter mechanisms, located either very close to the light source or very close to the sample.
U.S. Pat. No. 5,400,116 to Jehan et al teaches a sensitometer which includes a blade-type shutter in combination with a rotating shutter member having an aperture therein. The exposure plane is formed at one surface of an aperture plate which is moveable parallel to the sample and which contains an aperture through which light travels to expose the photographic sample. The size of this aperture determines the area of the sample to receive the exposure. The sample is loaded into a support means which moves the sample in the exposure plane. Gradated exposures are achieved by spinning the rotating shutter member at the speed necessary to produce the required exposure times. The slower operating blade-type shutter is used to prevent multiple exposures from occurring. If the required exposure time is sufficiently long for the blade-type shutter to operate, the blade-type shutter may be used to produce the exposure times with the rotating shutter member fixed so as to pass radiant energy therethrough.
U.S. Pat. No. 1,382,272 to Davis teaches a sensitometer which has an oscillating cap shutter and rotating sector shutter. The cap shutter can be set to remain open for an integer number of revolutions of the sector shutter. The sector shutter includes a pair of openings therein, one of which is graduated.
U.S. Pat. No. 1,883,884 to Durand teaches a photographic sensitometer which includes a rotating shutter member having an aperture therethrough a second plate mounted to the same shaft as the shutter member may be rotated relative to the shutter member to thereby vary the aperture supplied by the arcuate opening in the shutter member.
U.S. Pat. No. 2,763,192 to Tyler et al teaches a photographic sensitometer with a rotating shutter system. The shutter system includes a disk having a scanning slit therein. The length of the scanning slit can be adjusted to give different times of exposure by means of a second disk which is rotatably mounted to the face of the first disk. The speed of rotation of the first disk and the length of the scanning slit determine the time of exposure for each step of the film strip. Located behind the first disk is a pivoting shutter which is swung to a closed position in order to cover the window to the lamp house when the drive motor is not in operation. This pivoting shutter is operated by a rotary solenoid.
The prior art fails to teach a photographic sensitometer which uses both a rotating high speed sector shutter and a rotating capping shutter. Typically, prior art systems include a capping shutter or an electronic shutter to prevent the high speed sector shutter from making exposures every revolution (or every half revolution if the shutter has two apertures). These prior art capping shutters are essentially reciprocating blades. To make a complete cycle from closed to open to closed, the blades must be accelerated from rest (in the closed position) and decelerated to rest again (in the open position) and then accelerated and decelerated in the opposite direction back to the closed position again. An exemplary shutter of this type that has a sufficiently large aperture to allow light to pass is the Uniblitz.TM. VS25 as manufactured by Vincent Associates of Rochester, N.Y. The minimum time required to open and close the shutters is 10 msec. Further, there is an approximate 3 msec delay when power is supplied to the shutter to the time when the shutter actually starts moving. In addition, these times are valid only for an 80% opening and 20% closing. Longer times (unspecified by the manufacturer) are required in order to achieve a fully opened or fully closed position. Further, reciprocating blades do not allow for uniform exposure. That is, the center of the shutter is always open for a longer time than the edges of the shutter. In addition, reciprocating shutter systems tend to wear resulting in great differences in time to complete a cycle in what are otherwise identical reciprocating shutters.